1. Field of the Invention
The present invention relates to a charge transfer image pick-up device and a method of manufacturing the same and, more particularly, to an image pick-up device of this type including vertical registers and a horizontal register which are constituted with buried channel charge coupled devices (CCDs).
2. Description of the Prior Art
Although various proposals of a matrix type solid state image pick-up device had been reported in the early part of 1960s, none of them was realized. In 1970s, there was a considerable innovation in solid state image pick-up device owing to the development of MOS LSI technology and the development of charge transfer device and CCD. As a result, photo-electric conversion elements, charge storage elements and charge read-out elements which are to be arranged correspondingly to a number of pixels necessary for an image pick-up device were manufactured as LSI chips.
In 1980s, various characteristics thereof were considerably improved causing them to be practically usable and various applications of the solid state image pick-up device were also developed. Particularly, a CCD type image pick-up device utilizing charge transfer system was the most desirable device and an application thereof to HDTV (High Definition TV) is currently realized.
As disclosed in Extended Abstracts of the 1991 Internal Conference on Solid State Devices and Materials, Yokohama, 1991, p. 666 to 668, FIGS. 1 and 3, a CCD solid state image pick-up device is constituted basically with a plurality of vertical registers of CCDs, photo-electric converters arranged adjacent to respective registers constituting the vertical registers, transfer gates for controlling signal charge transfer from the photo-electric converters to corresponding vertical registers, respectively, a horizontal register electrically coupled to one ends of the vertical registers and a charge detector provided at one end of the horizontal register.
Such construction has been realized by forming, on a semiconductor substrate of one conductivity type, a well layer of the other conductivity type, forming, in the well layer, a buried layer of the one conductivity type and forming, on a main surface of the semiconductor layer, transfer electrodes of the vertical registers and transfer electrodes of the horizontal register on a gate insulating film covering the semiconductor substrate.
In general, in a buried channel type CCD, there is a tendency that the higher the impurity density of the well layer and the shallower the buried layer provides the larger the maximum amount of charge transfer. On the other hand, there is also a tendency that the lower the impurity density of the well layer provides the stronger the FIGURE electric field in the transfer direction and hence the higher the transfer efficiency.
However, since, in the conventional CCD type solid state image pick-up device mentioned above, the buried layer and the well layer are commonly used in both the vertical registers and the horizontal register, it is impossible to select such an impurity concentration that maximum amount of charge transfer of the vertical registers is maintained while improving the transfer efficiency of the horizontal register. Therefore, maximum amount of signal charge can not be obtained for the vertical registers and the transfer efficiency is degraded for the horizontal register, resulting in degradation of image quality.